1. Field of the Invention
This invention relates to an electrophotographic copying process for producing a plurality of copies from the same electrostatic latent image of an original formed on an electrostatic charge retentive member, by repeating development of the electrostatic latent image and transfer of the thus developed image to image receiving members.
2. Description of the Prior Art
To obtain a plurality of high quality copies from the same electrostatic latent image formed on an electrostatic charge retentive member, it is important to maintain the stable electrostatic latent image of the original on the electrostatic charge retentive member, such as a rotary photosensitive drum, throughout the copy production cycle. In practice, however, the electrostatic charge leaks from the electrostatic latent image for various reasons, so that it is very difficult to ensure high picture quality throughout a plurality of copies made by the repeated development and transfer of the one latent image. One of the causes of such deterioration of the electrostatic latent image is the transfer of electrostatic charge during the development. Such electrostatic charge transfer is due to leakage of the electrostatic charge of the latent image from the electrostatic charge retentive member through the developer or injection of unwanted electrostatic charge onto the electrostatic charge retentive member through the developer. More specifically, it has been known that such deterioration of the electrostatic latent image tends to occur in the case of a magnetic brush developing unit which uses a combination of two-component developer containing magnetic carrier and toner and an electrically-conductive non-magnetic sleeve such as an aluminium sleeve. It is believed that the magnetic carrier particles are electrically conductive and allow the leakage and injection of electrostatic charge.
Several proposals have been made to overcome the shortcoming. For instance, one proposal is to insulate the surfaces of the magnetic carrier particles by coating the surfaces with resin or the like. This proposal is effective but has shortcomings in that the process of producing the magnetic carrier is complicated by the need for special insulating treatment. Further the process is expensive because of the need for special equipment to accomplish the insulating treatment.
Another proposal is to provide an electric insulating layer on the surface of the non-magnetic sleeve. This proposal can be effected comparatively easily at relatively low cost but has a shortcoming in that, in the case of copying a solid picture, front edges of the copied picture tend to lose picture density as the number of copies increases. Despite that, high picture quality can be maintained from the first copy to the last copy in the case of copying a linear picture, as confirmed by experiments.
The decrease in print density at the front edge of the solid picture may be mitigated by increasing the developing efficiency, e.g. by increasing the toner concentration. However, in such a case the linear picture might be over-developed and might become thick. Further, the background of the image might be undesirously developed to some extent. In this connection it should be noted that the single document image may include both the solid picture and the linear picture. Therefore, in the known process it is very difficult to develop both the linear picture and the solid picture in an optimum manner.